Optical bodies including strippable boundary layers

ABSTRACT

The present disclosure is directed to optical bodies including a first optical film, a second optical film and one or more strippable boundary layers disposed between the first and second optical films. Each major surface of a strippable boundary layer may be disposed adjacent to an optical film or another strippable boundary layer. At least one of the first and second optical films may include a reflective polarizer. The present disclosure is also directed to methods of processing such optical bodies.

FIELD OF THE INVENTION

The present disclosure relates to optical bodies and methods ofprocessing optical bodies.

BACKGROUND

Optical films, including polymeric single layer optical films, polymericmultilayer optical films and polymeric optical films including disperseand continuous phases, are widely used for various purposes. Exemplaryapplications of polymeric optical films include display devices, such asliquid crystal displays (LCDs) placed in mobile telephones, personaldata assistants, computers, televisions and other devices. Well knownpolymeric optical films include reflective polarizer films, such asVikuiti™ Dual Brightness Enhancement Film (DBEF) and Vikuiti™ DiffuseReflective Polarizer Film (DRPF), both available from 3M Company. Otherwell known polymeric optical films include reflectors, such as Vikuiti™Enhanced Specular Reflector (ESR), also available from 3M Company.

Polymeric multilayer optical films used as polarizers or mirrors,usually include one or more first optical layers and one or more secondoptical layers. In addition to the first and second optical layers, sometraditional multilayer films include one or more non-optical layers,such as one or more protective boundary layers located over or betweenpackets of optical layers. The non-optical layers are usually integratedinto the polymeric multilayer optical films so that at least a portionof the light to be transmitted, polarized, or reflected by the first andsecond optical layers also travels through these non-optical layers.Such non-optical layers can protect the optical layers from damage, aidin co-extrusion processing and/or enhance post-processing mechanicalproperties of the optical films. Thus, in such traditional opticalfilms, it is usually important that the non-optical layers do notsubstantially affect the reflective properties of the optical films overthe wavelength region of interest.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed to optical bodiesincluding a first optical film, a second optical film and one or morestrippable boundary layers disposed between the first and second opticalfilms. Each major surface of a strippable boundary layer is disposedadjacent to an optical film or another strippable boundary layer. Atleast one of the first and second optical films may include a reflectivepolarizer.

In another aspect, the present disclosure is directed to methods forprocessing optical bodies, which include providing an optical bodycomprising a first optical film, a second optical film and at least onestrippable boundary layer disposed between the first and second opticalfilms. The optical body is conveyed into a stretching region andstretched to increase a transverse dimension of the optical body whileconveying the opposing edges of the optical body along generallydiverging paths in a machine direction. In this exemplary embodiment,the generally diverging paths are configured and arranged to provide amachine direction draw ratio (MDDR), a normal direction draw ratio(NDDR) and a transverse direction draw ratio (TDDR) that approach thefollowing relationship:MDDR=NDDR=(TDDR)^(−1/2) during the stretching.

In another aspect, the present disclosure is directed to methods ofprocessing optical bodies, which include providing an optical bodycomprising a first optical film, a second optical film and at least onestrippable boundary layer disposed between the first and second opticalfilms. The optical body is conveyed within a stretcher along a machinedirection while holding opposing edge portions of the optical body andstretched to a draw ratio in excess of four within the stretcher bymoving the opposing edge portions along diverging non-linear paths. Inthis exemplary embodiment, during the stretching, the minimum value ofthe extent of uniaxial character, U, is at least 0.7 over a finalportion of the stretching after achieving a TDDR of 2.5 and U is lessthan 1 at the end of the stretching, wherein U is defined asU=(1/MDDR−1)/(TDDR^(1/2)−1) wherein MDDR is the machine direction drawratio and TDDR is the transverse direction draw ratio as measuredbetween the diverging paths.

In yet another aspect, the present disclosure is directed to methods ofprocessing optical bodies, which include providing an optical bodycomprising a first optical film, a second optical film and at least onestrippable boundary layer disposed between the first and second opticalfilms. The optical body is conveyed within a stretcher along a machinedirection while holding opposing edge portions of the optical body andstretched by moving the opposing edge portions along divergingnon-linear paths. In this exemplary embodiment, during the stretching ofthe optical body, the speed of the film along the machine directiondecreases by a factor of approximately λ^(1/2) where λ is the transversedirection draw ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those of ordinary skill in the art to which the subjectinvention pertains will more readily understand how to make and use thesubject invention, exemplary embodiments thereof are described in detailbelow with reference to the drawings, wherein:

FIG. 1 is a schematic partial cross-sectional view of an optical bodyconstructed in accordance with an exemplary embodiment of the presentdisclosure;

FIG. 2 is a schematic partial cross-sectional view of an optical bodyconstructed in accordance with another exemplary embodiment of thepresent disclosure;

FIG. 3 is a schematic partial cross-sectional view of an optical bodyconstructed in accordance with yet another embodiment of the presentdisclosure;

FIG. 4 illustrates uniaxially stretching an optical body; and

FIG. 5 is a schematic top view of an apparatus that may be used forprocessing optical bodies according to the present disclosure.

DETAILED DESCRIPTION

As summarized above, the present disclosure provides an optical bodythat includes one or more strippable boundary layers and methods ofmaking such optical bodies. According to the principles of the presentdisclosure, each strippable boundary layer is connected to at least oneoptical film. In some exemplary embodiments, one or more strippableboundary layers can be made rough and used to impart a surface textureinto one or more optical films, for example, by co-extruding ororienting the optical film or films with a rough strippable boundarylayer or by other suitable methods. The one or more rough strippableboundary layers can be constructed and used substantially in the samemanner as rough strippable skin layers described in the commonly ownedU.S. application Ser. No. 10/977,211 to Hebrink et al., entitled“Optical Bodies and Methods for Making Optical Bodies”, filed on Oct.29, 2004, the disclosure of which is hereby incorporated by referenceherein to the extent it is not inconsistent with the present disclosure.

In typical embodiments of the present disclosure, the strippableboundary layers are connected to one or more optical films, such thatthey are capable of remaining adhered to the one or more optical filmsduring initial processing, such as stretching, or in some exemplaryembodiments, also during subsequent storage, handling, packaging,transporting and/or conversion, but can be stripped or removed by a userwhen desired. For example, the strippable boundary layers can be removedand the optical films separated shortly after stretching the opticalbody or shortly prior to installation of one or more of the constituentoptical films into a display device. Preferably, the one or morestrippable boundary layers and the one or more optical films areseparated without applying excessive force, damaging the optical films,or contaminating the optical films with a substantial residue ofparticles from the strippable boundary layers. In other exemplaryembodiments, optical bodies of the present disclosure may be installedinto a display device with at least one strippable boundary layer stillintact. This feature provides additional flexibility as to the form inwhich the optical bodies of the present disclosure may be used.

Reference is now made to FIGS. 1, 2 and 3 showing exemplary embodimentsof the present disclosure in simplified schematic form. FIG. 1 is apartial schematic cross-sectional view showing an optical body 10, whichincludes a first optical film 20, a second optical film 30 and at leastone strippable boundary layer 18 disposed between the first and secondoptical films. A first surface of the strippable boundary layer may bedisposed adjacent to the optical film 20 and a second surface of thestrippable boundary layer may be disposed adjacent to the optical film30. In other exemplary embodiments, the strippable boundary layer may bedisposed adjacent to one optical film and separated from another opticalfilm by an additional layer, which may be an additional strippable layeror layers or the additional layer may be attached to the adjacentoptical film. When desired, two strippable boundary layers may beprovided between the optical films 20 and 30, for example, to providedifferent amounts of adhesion of a strippable boundary layer to theadjacent optical film 20 or 30. The optical body 10 may optionallyfurther include one or more additional strippable boundary layers 18disposed at outer surfaces of the optical films 20 and 30 and disposedadjacent to only one optical film, and one or more outer skin layers 16.

One example of materials that can be advantageously used in aconstruction shown in FIG. 1 is as follows: (1) first optical layers 12made of 55 mol % of a diacid such as naphthalene dicarboxylate, 45% mol% of a diacid such as dimethyl terephthalate, 4 mol % hexane diol in thediol and 96 mol % ethlyene glycol; (2) second optical layers 14 made ofpolyethylene napthalate; (3) strippable boundary layers 18 made ofpolypropylene; and (3) outer skin layers 16 made of 75 mol % of a diacidsuch as naphthalene dicarboxylate, 25% mol % of a diacid such asdimethyl terephthalate, 4 mol % hexane diol in the diol and 96 mol %ethlyene glycol.

Exemplary optical bodies including more than two optical films may alsofurther include additional strippable boundary layers (not shown)disposed between first and second optical films 20 and 30 and suchadditional optical films or between additional optical films (notshown). For example, the optical body 10 may further include a thirdoptical film disposed next to the second optical film and a secondstrippable boundary layer disposed between the second optical film andthe third optical film. Other exemplary embodiments may include morethan three optical films, e.g., 6, 10 or more. The number of opticalfilms used in an optical body constructed according to the presentdisclosure will depend on the equipment and materials used, as well asother relevant factors. Furthermore, the optical body 10 may include anyother additional layers when suitable for a particular application. Forexample, one or both optical films 20 and 30 may further include one ormore under-skin layers disposed between the optical film and thestrippable boundary layer and forming a part of the optical film.

In some exemplary embodiments, one of or both optical films 20 and 30may be or may include polymeric multilayer optical films, such asmultilayer reflective polarizers. For example, one or both optical filmsmay include one or more first optical layers 12 and one or more secondoptical layers 14. The first optical layers 12 may be birefringentpolymer layers that are uniaxially or biaxially oriented. The secondoptical layers 14 may also be polymer layers that are birefringent anduniaxially or biaxially oriented. In other exemplary embodiments, thesecond optical layers 14 have an isotropic index of refraction that isdifferent from at least one of the indices of refraction of the firstoptical layers 12 after orientation. One or both of the optical films 20and 30 may be or may include polymeric optical films including adisperse phase and a continuous phase, such as a diffuse reflectivepolarizer. In yet other exemplary embodiments, one or more of theoptical films 20 and 30 may be single-layer optical films.

FIG. 2 shows a partial schematic cross-sectional view of an optical body40 constructed according to another exemplary embodiment of the presentdisclosure. The optical body 40 includes a first optical film 50, asecond optical film 60 and a strippable boundary layer 48 disposedbetween the first and second optical films 50 and 60. In this exemplaryembodiment, the strippable boundary layer 48 is a rough strippableboundary layer including a continuous phase 47 and a disperse phase 49.The disperse phase 49 can be formed by blending particles in thecontinuous phase 47 or by mixing in a material or materials that areimmiscible in the continuous phase 47 at the appropriate stages ofprocessing, which preferably then phase-separate and form a roughsurface at the interface between the strippable boundary layer materialand the optical film. For some applications, it may be desirable to forma boundary layer with one or more layers having continuous and dispersephases in which the interface between the two phases will besufficiently weak to result in voiding when the film is oriented orotherwise processed. Such voids can contribute into creating the roughinterface between the boundary layer and the adjacent optical film. Theaverage dimensions and aspect ratio of the voids may be controlledthrough careful manipulation of processing parameters and stretchratios, or through selective use of compatibilizers.

The continuous phase 47 and disperse phase 49 are shown in a generalizedand simplified view in FIG. 2, while in practice the two phases can beless uniform and more irregular in appearance. For example, theschematic representation in FIG. 2 will be understood to cover theembodiment in which the strippable boundary layer includes a firstpolymer and a second polymer that is substantially immiscible in thefirst polymer but does not form clearly dispersed regions. In someexemplary embodiments, the strippable boundary layer 48 may containmultiple sub-phases of the disperse or/and the continuous phase. Thestrippable boundary layer 48 can be used to impart a surface textureincluding depressions 50 a into a surface of the optical film 50 that isdisposed adjacent to the strippable boundary layer 48, and a surfacetexture including depressions 60 a into a surface of the optical film 60that is disposed adjacent to the strippable boundary layer 48. Thesurface texture can be thus imparted during coextrusion, laminationand/or subsequent stretching of the optical films with the strippableboundary layers. The optical body 40 may further include any number offilms or layers shown or described in reference to FIG. 1 and any otheradditional layers when suitable for a particular application.

FIG. 3 shows a partial schematic cross-sectional view of an optical body70 constructed according to yet another exemplary embodiment of thepresent disclosure. The optical body 70 includes a first optical film80, a second optical film 90, a strippable boundary layer 78 including acontinuous phase 77 and a disperse phase 79 and a smooth strippableboundary layer 75, which can be formed integrally and removed with therough strippable boundary layer 78. Alternatively, the smooth strippableboundary layer 75 can be formed and/or removed separately from the roughstrippable boundary layer 78. In some exemplary embodiments, the smoothboundary layer 75 can include at least one of the same materials as thecontinuous phase 77.

The strippable boundary layer 48 can be used to impart a surface textureincluding depressions 50 a into a surface of the optical film 50 that isdisposed adjacent to the strippable boundary layer 48, and a surfacetexture including depressions 60 a into a surface of the optical film 60that is disposed adjacent to the strippable boundary layer 48. Thesurface texture can be thus imparted during the coextrusion, laminationand/or subsequent stretching of the optical films with the strippableboundary layers. When desired, two rough strippable layers may beprovided between the optical films 80 and 90, for example, withdifferent amounts of disperse phase to impart different amounts ofroughness into different optical films. Furthermore, the optical body 70may include any number of films or layers shown or described inreference to FIGS. 1 and 2 and any other additional layers when suitablefor a particular application.

Strippable boundary layers included into the optical bodies constructedaccording to the present disclosure may have a first major surface thatis removably attached to a first optical film and a second major surfacethat is removably attached to a second optical film. However, someexemplary optical bodies constructed according to the present disclosuremay include at least one boundary layer that has a first major surfacethat is removably attached to a first optical film and a second majorsurface that is permanently attached to a second optical film via amaterial selection that provides an acceptable bond between the boundarylayer and the second optical film so that the strippable boundary layermay be removed from the first optical film but not from the secondoptical film. In some embodiments, one of the optical films may serve asa skin layer added to satisfy processing requirements (coextrusionprocess or film handling and/or converting), which can be removed atsome point of the process and discarded.

In another embodiment, a boundary layer may adhere to both the first andsecond optical films and, upon stripping, it may split to produceadditional layers on the first and second optical films that arecomposed of boundary layer material. One way of obtaining these effectsis to have a boundary layer that is a multilayer material composed oftwo or more materials, as explained above. In some of such exemplaryembodiments that material selection will include material that havestronger or weaker adhesion to the adjacent optical film. The selectionof these materials will be governed by the material composition of theadjacent optical film.

The optical films and layers depicted in FIGS. 1, 2 and 3 can beconstructed to have different relative thicknesses than thoseillustrated.

Additional aspects of the invention will now be explained in greaterdetail.

Optical Films

Various optical films are suitable for use in the embodiments of thepresent disclosure. Optical films suitable for use in some embodimentsof the present disclosure can include dielectric multilayer opticalfilms (whether composed of all birefringent optical layers, somebirefringent optical layers, or all isotropic optical layers), such asDBEF and ESR, and continuous/disperse phase optical films, such as DRPF,which can be characterized as polarizers or mirrors. Optical filmssuitable for use in embodiments of the present disclosure can be or caninclude a diffuse micro-voided reflective film, such as BaSO4-filledPET, or diffuse “white” reflective film such as TiO₂-filled PET.Alternatively, the optical film can be a single layer of a suitableoptically clear isotropic or birefringent material, e.g., polycarbonate,and it may or may not include volume diffusers. Those of ordinary skillin the art will readily appreciate that the structures, methods, andtechniques described herein can be adapted and applied to other types ofsuitable optical films. The optical films specifically mentioned hereinare merely illustrative examples and are not meant to be an exhaustivelist of optical films suitable for use with exemplary embodiments of thepresent disclosure.

More particularly, exemplary optical films that are suitable for use inembodiments of the present disclosure include multilayer reflectivefilms such as those described in, for example, U.S. Pat. Nos. 5,882,774and 6,352,761 and in PCT Publication Nos. WO95/17303; WO95/17691;WO95/17692; WO95/17699; WO96/19347; and WO99/36262, all of which areincorporated herein by reference. Both multilayer reflective polarizeroptical films and continuous/disperse phase reflective polarizer opticalfilms rely on index of refraction differences between at least twodifferent materials (typically polymers) to selectively reflect light ofat least one polarization orientation. Suitable diffuse reflectivepolarizers include the continuous/disperse phase optical films describedin, for example, U.S. Pat. No. 5,825,543, incorporated herein byreference, as well as the diffusely reflecting optical films describedin, for example, U.S. Pat. No. 5,867,316, incorporated herein byreference. Other materials and optical films including a disperse phaseand a continuous phase, suitable for use in some embodiments of thepresent disclosure are also described in a commonly owned applicationentitled “Diffuse Reflective Polarizing Films With Orientable PolymerBlends,” 3M Docket No. 60758US002, filed on even date herewith, thedisclosure of which is hereby incorporated by reference herein to theextent it is not inconsistent with the present disclosure.

In some embodiments, one or more of the optical films is a multilayerstack of polymer layers with a Brewster angle (the angle at whichreflectance of p-polarized light turns to zero) that is very large ornonexistent. As it is known by those of ordinary skill in the art,multilayer optical films can be made into a multilayer mirror orpolarizer whose reflectivity for p-polarized light decreases slowly withangle of incidence, is independent of angle of incidence, or increaseswith angle of incidence away from the normal. Multilayer reflectiveoptical films are used herein as an example to illustrate optical filmstructures and methods of making and using the optical films of theinvention. As mentioned above, the structures, methods, and techniquesdescribed herein can be adapted and applied to other types of suitableoptical films.

For example, a suitable multilayer optical film can be made byalternating (e.g., interleaving) uniaxially- or biaxially-orientedbirefringent first optical layers with second optical layers. In someembodiments, the second optical layers have an isotropic index ofrefraction that is approximately equal to one of the in-plane indices ofthe oriented layer. The interface between the two different opticallayers forms a light reflection plane. Light polarized in a planeparallel to the direction in which the indices of refraction of the twolayers are approximately equal will be substantially transmitted. Lightpolarized in a plane parallel to the direction in which the two layershave different indices will be at least partially reflected. Thereflectivity can be increased by increasing the number of layers or byincreasing the difference in the indices of refraction between the firstand second layers.

A film having multiple layers can include layers with different opticalthicknesses to increase the reflectivity of the film over a range ofwavelengths. For example, a film can include pairs of layers that areindividually tuned (for normally incident light, for example) to achieveoptimal reflection of light having particular wavelengths. Generally,multilayer optical films suitable for use with certain embodiments ofthe invention have about 2 to 5000 optical layers, typically about 25 to2000 optical layers, and often about 50 to 1500 optical layers or about75 to 1000 optical layers. Some exemplary embodiments include about 825optical layers or less, about 600 optical layers or less, about 275layers or less, or even about 100 optical layers or less. The number ofoptical layers depends on the application. It should further beappreciated that, although only a single multilayer stack may bedescribed, the multilayer optical film can be made from multiple stacksor different types of optical film that are subsequently combined toform the film.

A reflective polarizer can be made by combining a uniaxially orientedfirst optical layer with a second optical layer having an isotropicindex of refraction that is approximately equal to one of the in-planeindices of the oriented layer. Alternatively, both optical layers areformed from birefringent polymers and are oriented in a draw process sothat the indices of refraction in a single in-plane direction areapproximately equal. The interface between the two optical layers formsa light reflection plane for one polarization of light. Light polarizedin a plane parallel to the direction in which the indices of refractionof the two layers are approximately equal will be substantiallytransmitted. Light polarized in a plane parallel to the direction inwhich the two layers have different indices will be at least partiallyreflected.

For polarizers having second optical layers with isotropic indices ofrefraction or low in-plane birefringence (e.g., no more than about 0.07at 632.8 nm), the in-plane indices (n_(x) and n_(y)) of refraction ofthe second optical layers are approximately equal to one in-plane index(e.g., n_(y)) of the first optical layers. Thus, the in-planebirefringence of the first optical layers is an indicator of thereflectivity of the multilayer optical film. Typically, it is found thatthe higher the in-plane birefringence, the better the reflectivity ofthe multilayer optical film. Typically, the first optical layers have anin-plane birefringence (n_(x)−n_(y)) after orientation of about 0.04 orgreater at 632.8 nm, about 0.1 or greater at 632.8 nm, about 0.15 orgreater at 632.8 nm, preferably about 0.2 or greater at 632.8 nm, andmore preferably about 0.3 or greater at 632.8 nm. If the out-of-planeindices (n_(z)) of refraction of the first and second optical layers areequal or nearly equal, the multilayer optical film also has betteroff-angle reflectivity. The same or similar design considerations applyto diffuse reflective polarizers including disperse and continuouspolymeric phases.

A mirror can be made using at least one uniaxially birefringentmaterial, in which two indices (typically along the x and y axes, orn_(x) and n_(y)) are approximately equal, and different from the thirdindex (typically along the z axis, or n_(z)). The x and y axes aredefined as the in-plane axes, in that they represent the plane of agiven layer within the multilayer film, and the respective indices n_(x)and n_(y) are referred to as the in-plane indices. One method ofcreating a uniaxially birefringent system is to biaxially orient(stretch along two axes) the multilayer polymeric film. If the adjoininglayers have different stress-induced birefringence, biaxial orientationof the multilayer film results in differences between refractive indicesof adjoining layers for planes parallel to both axes, resulting in thereflection of light of both planes of polarization.

Where the first optical layers are birefringent polymer layers that areuniaxially- or biaxially-oriented, the polymers of the first opticallayers are typically selected to be capable of developing a largebirefringence when stretched. Depending on the application, thebirefringence may be developed between two orthogonal directions in theplane of the film, between one or more in-plane directions and thedirection perpendicular to the film plane, or a combination of these.The first polymer should maintain birefringence after stretching, sothat the desired optical properties are imparted to the finished film.The second optical layers can be polymer layers that are birefringentand uniaxially- or biaxially-oriented, or the second optical layers canhave an isotropic index of refraction that is different from at leastone of the indices of refraction of the first optical layers afterorientation. In the latter case, the polymer of the second layers shoulddevelop little or no birefringence when stretched, or developsbirefringence of the opposite sense (positive-negative ornegative-positive), such that its film-plane refractive indices differas much as possible from those of the polymer of the first opticallayers in the finished film.

Materials suitable for making optical films for use in exemplaryembodiments of the present disclosure include polymers such as, forexample, polyesters, copolyesters and modified copolyesters. In thiscontext, the term “polymer” will be understood to include homopolymersand copolymers, as well as polymers or copolymers that may be formed ina miscible blend, for example, by co-extrusion or by reaction,including, for example, transesterification. The terms “polymer” and“copolymer” include both random and block copolymers.

Exemplary polymers useful in the optical films of the present disclosureinclude polyethylene naphthalate (PEN). PEN is frequently chosen for usein the first optical layers. Other polymers suitable for use in thefirst optical layers include, for example, polybutylene 2,6-naphthalate(PBN), polyethylene terephthalate (PET), and copolymers thereof. Othermaterials suitable for use in optical films and, particularly, in thefirst optical layers, are described, for example, in U.S. Pat. Nos.5,882,774, 6,352,761 and 6,498,683 and U.S. patent application Ser. Nos.09/229,724, 09/232,332, 09/399,531, and 09/444,756, which areincorporated herein by reference. An exemplary coPEN suitable for use inthe first optical layers is coPEN having carboxylate subunits derivedfrom 90 mol % dimethyl naphthalene dicarboxylate and 10 mol % dimethylterephthalate and glycol subunits derived from 100 mol % ethylene glycolsubunits and an intrinsic viscosity (IV) of 0.48 dL/g. Another usefulpolymer is a PET having an intrinsic viscosity of 0.74 dL/g, availablefrom Eastman Chemical Company (Kingsport, Tenn.).

Polymer or polymers suitable for use in the second optical layers shouldbe chosen so that in the finished film, the refractive index, in atleast one direction, differs significantly from the index of refractionof the first optical layers in the same direction. In addition, it willbe understood that the choice of a second polymer is dependent not onlyon the intended application of the optical film in question, but also onthe choice made for the first polymer, as well as processing conditions.

The second optical layers can be made from a variety of polymers havingglass transition temperatures compatible with that of the first opticallayers and having a refractive index similar to the isotropic refractiveindex of the first polymer. Examples of other polymers suitable for usein optical films and, particularly, in the second optical layers, otherthan the coPEN polymers mentioned above, include vinyl polymers andcopolymers made from monomers such as vinyl naphthalenes, styrene,maleic anhydride, acrylates, and methacrylates. Examples of suchpolymers include polyacrylates, polymethacrylates, such as poly (methylmethacrylate) (PMMA), and isotactic or syndiotactic polystyrene. Otherpolymers include condensation polymers such as polysulfones, polyamides,polyurethanes, polyamic acids, and polyimides. In addition, the secondoptical layers can be formed from polymers and copolymers such aspolyesters and polycarbonates.

Other exemplary suitable polymers, especially for use in the secondoptical layers, include homopolymers of polymethylmethacrylate (PMMA),such as those available from Ineos Acrylics, Inc., Wilmington, Del.,under the trade designations CP71 and CP80, or polyethyl methacrylate(PEMA), which has a lower glass transition temperature than PMMA.Additional second polymers include copolymers of PMMA (coPMMA), such asa coPMMA made from 75 wt % methylmethacrylate (MMA) monomers and 25 wt %ethyl acrylate (EA) monomers, (available from Ineos Acrylics, Inc.,under the trade designation Perspex CP63), a coPMMA formed with MMAcomonomer units and n-butyl methacrylate (nBMA) comonomer units, or ablend of PMMA and poly(vinylidene fluoride) (PVDF) such as thatavailable from Solvay Polymers, Inc., Houston, Tex. under the tradedesignation Solef 1008.

Yet other suitable polymers, especially for use in the second opticallayers, include polyolefin copolymers such as poly (ethylene-co-octene)(PE-PO) available from Dow-Dupont Elastomers under the trade designationEngage 8200, poly (propylene-co-ethylene) (PPPE) available from Fina Oiland Chemical Co., Dallas, Tex., under the trade designation Z9470, and acopolymer of atatctic polypropylene (aPP) and isotatctic polypropylene(iPP). The optical films can also include, for example in the secondoptical layers, a functionalized polyolefin, such as linear low densitypolyethylene-g-maleic anhydride (LLDPE-g-MA) such as that available fromE.I. duPont de Nemours & Co., Inc., Wilmington, Del., under the tradedesignation Bynel 4105.

Exemplary combinations of materials in the case of polarizers includePEN/co-PEN, polyethylene terephthalate (PET)/co-PEN, PEN/sPS,PEN/Eastar, and PET/Eastar, where “co-PEN” refers to a copolymer orblend based upon naphthalene dicarboxylic acid (as described above) andEastar is polycyclohexanedimethylene terephthalate commerciallyavailable from Eastman Chemical Co. Exemplary combinations of materialsin the case of mirrors include PET/coPMMA, PEN/PMMA or PEN/coPMMA,PET/ECDEL, PEN/ECDEL, PEN/sPS, PEN/THV, PEN/co-PET, and PET/sPS, where“co-PET” refers to a copolymer or blend based upon terephthalic acid (asdescribed above), ECDEL is a thermoplastic polyester commerciallyavailable from Eastman Chemical Co., and THV is a fluoropolymercommercially available from 3M. PMMA refers to polymethyl methacrylateand PETG refers to a copolymer of PET employing a second glycol (usuallycyclohexanedimethanol). sPS refers to syndiotactic polystyrene.Non-polyester polymers may be used in creating polarizer films. Forexample, polyether imides can be used with polyesters, such as PEN andcoPEN, to generate a multilayer reflective mirror. Otherpolyester/non-polyester combinations, such as polyethylene terephthalateand polyethylene (e.g., those available under the trade designationEngage 8200 from Dow Chemical Corp., Midland, Mich.), can be used.

Optical films included in the optical bodies constructed according tothe present disclosure are typically thin, but in other exemplaryembodiments they may be as thick as desired. Suitable films may havevarious thicknesses, but usually they include films with thicknesses ofless than 15 mils (about 380 micrometers), typically less than 10 mils(about 250 micrometers), more typically less than 7 mils (about 180micrometers), sometimes, less than 5 mils, less than 1.5 mils, or evenless than 1 mil, e.g., 0.7 mils. During processing, a dimensionallystable layer may be included into the optical film by extrusion coatingor coextrusion. Optical films of the present disclosure can also includeoptional other optical or non-optical layers, such as one or morenon-strippable protective boundary layers between packets of opticallayers. The non-optical layers may be of any appropriate materialsuitable for a particular application and can be or can include at leastone of the materials used in the remainder of the optical film.

In some exemplary embodiments, an intermediate layer or an underskinlayer can be integrally formed with the optical film or on one or moreof its outer surfaces. One or more under-skin layers are typicallyformed by co-extrusion with the optical film, for example, to integrallyform and bind the first and second optical layers. The underskin layeror layers can include immiscible blends with a continuous phase and adisperse phase which also can aid in creating surface roughness andhaze. The disperse phase of the underskin layers can be polymeric orinorganic and, where a substantially clear optical film is desired, haveabout the same or similar refractive index as the continuous phase. Insome exemplary embodiments of such clear optical films, the refractiveindexes of the materials making up the disperse and continuous phasesdiffer from each other by no more than about 0.02. An example ofunderskin layer with refractive index matched blend is a continuousphase comprising SAN and a disperse phase comprising PETG (copolyestercommercially available from Eastman Chemical under the tradename Eastar6763). An example of underskins with a refractive index mismatched blendis a continuous phase of Xylex 7200 and a disperse phase of polystyrene.

Strippable Boundary Layers

By selecting the materials comprised in the one or more strippableboundary layers, the interfacial adhesion between the strippableboundary layer(s) and the adjacent optical film can be controlled sothat the strippable boundary layers are capable of remaining adhered tothe optical film(s) for as long as desired for a particular application,but can also be cleanly stripped or removed from the optical film(s)before use without applying excessive force or, in the appropriateembodiments, without leaving a substantial residue of particles from theboundary layer on the adjacent optical film.

In some exemplary embodiments of the present disclosure, the materialscomprised in the optical bodies with the strippable boundary layer(s)connected to the optical film(s) are substantially transparent or clear,so that the optical bodies can be inspected for defects using standardinspection equipment. Such exemplary clear optical bodies usually havestrippable boundary layers in which the constituent materials haveapproximately the same or sufficiently similar refractive indexes. Insome exemplary embodiments of such clear optical bodies, the refractiveindexes of the materials making up the strippable boundary layers differfrom each other by no more than about 0.02.

A boundary layer adhered to an adjacent surface of an optical film inexemplary optical bodies of the present disclosure, can be constructedso that the adhesion of the strippable boundary layer(s) to the opticalfilm(s) is characterized by a peel force of about 2 g/in or more betweena strippable boundary layer and the adjacent optical film. Otherexemplary optical bodies constructed according to the present disclosurecan be characterized by a peel force of about 4, 5, 10 or 15 g/in ormore. In some exemplary embodiments, the optical bodies can becharacterized by a peel force as high as about 100 g/in or even about120 g/in. In other exemplary embodiments, the optical bodies can becharacterized by a peel force of about 50, 35, 30 or 25 g/in or less. Insome exemplary implementations the adhesion can be in the range from 2g/in to 120 g/in, from 4 g/in to 50 g/in, from 5 g/in to 35 g/in, from10 g/in to 25 g/in, or from 15 g/in to 25 g/in. In other exemplaryembodiments, the adhesion can be within other suitable ranges. Peelforces over 120 g/in can be tolerated for some applications depending onthe materials used.

In some exemplary embodiments that are characterized by higher values ofpeel forces between a strippable boundary layer and an adjacent opticalfilm, various steps can be taken to aid in removal of the strippableboundary layers from one or more optical films. For example, an opticalbody of the present disclosure may be subjected to heat-setting,maintained at a particular temperature during the removal, subjected totension, or/and allowed to age, which may permit any lubricantscontained therein to reach a film or a layer surface.

The peel force that can be used to characterize exemplary embodiments ofthe present disclosure can be measured as follows. In particular, thepresent test method provides a procedure for measuring the peel forceneeded to remove a strippable boundary layer from an optical film (e.g.,multilayer film, polycarbonate, etc.). Test-strips are cut from theoptical body with a strippable boundary layer adhered to an opticalfilm. The strips are typically about 1″ width, and more than about 6″ inlength. The strips may be pre-conditioned for environmental agingcharacteristics (e.g., hot, hot & humid, cold, thermal-shock).

Typically, the samples should dwell for more than about 24 hours priorto testing. The 1″ strips are then applied to rigid plates, for example,using double-sided tape (such as Scotch™ double sided tape availablefrom 3M), and the plate/test-strip assembly is fixed in place on thepeel-tester platen. The leading edge of the strippable boundary layer isthen separated from the optical film and clamped to a fixture connectedto the peel-tester load-cell. The platen holding the plate/test-stripassembly is then carried away from the load-cell at constant speed ofabout 90 inches/minute, effectively peeling the strippable boundarylayer from the substrate optical film at about a 180 degree angle. Asthe platen moves away from the clamp, the force required to peel thestrippable boundary layer off the film is sensed by the load cell andrecorded by a microprocessor. The force required for peel is thenaveraged over 5 seconds of steady-state travel (preferably ignoring theinitial shock of starting the peel) and recorded.

It has been found that these and related goals can be accomplished bycareful selection of the materials for making the strippable boundarylayers and ensuring their compatibility with at least some of thematerials used to make the optical film, especially the materials of theouter surfaces of the optical film or, in the appropriate exemplaryembodiments, of the under-skin layers. In accordance with oneimplementation of the present disclosure, the strippable boundary layersmay include a sufficient amount of material with low crystallinity or anamorphous material, in order to remain adhered to the optical film for adesired period of time. In some exemplary embodiments, two or moredifferent materials with different adhesions can be used in thestrippable boundary layers to achieve a desired amount of adhesion.

Materials suitable for use in the strippable boundary layer(s) include,for example, fluropolymers such as polyvinylidene fluoride (PVDF),ethylene-tetrafluoroethylene fluoropolymers (ETFE),polytetrafluoroethylene (PTFE), copolymers of PMMA (or a coPMMA) andPVDF, or any of the THV or PFA materials available from 3M (St. Paul,Minn.). Processing aids such as Dynamar (available from 3M) or Glycolube(available fro Lonza Corporation in Fair Lawn N.J.) may enhance releasecharacteristics of strippable boundary layers.

Materials suitable for use in the strippable boundary layer(s) generallyinclude polyolefins, such as polypropylene and modified polypropylenes.Aliphatic polyolefins can be used. One suitable group of polypropylenesincludes high density polypropylenes which exhibit particularly lowadhesion to polyester and acrylic materials, and which are commonly usedto make multilayer optical films. Polyethylenes and their copolymers arealso may be useful, including copolymers and propylene and ethylene.Other exemplary materials include polymethylpentene, cyclic olefincopolymers such as Topas available from Ticona Engineering Polymers(Florence, Ky.), copolymers of olefins with maleic anhydride, acrylicacid, or glycidyl methacrylate, or any of the Hytrel (thermoplasticpolyester elastomer) or Bynel (modified ethylene vinyl acetate)materials available from DuPont Corporation (Wilmington, Del.).

Syndiotactic and atactic Vinyl aromatic polymers, which may be useful insome embodiments of the present disclosure, include poly(styrene),poly(alkyl styrene), poly(styrene halide), poly(alkyl styrene),poly(vinyl ester benzoate), and these hydrogenated polymers andmixtures, or copolymers containing these structural units. Examples ofpoly(alkyl styrenes) include: poly(methyl styrene), poly(ethyl styrene),poly(propyl styrene), poly(butyl styrene), poly(phenyl styrene),poly(vinyl naphthalene), poly(vinylstyrene), and poly(acenaphthalene)may be mentioned. As for the poly(styrene halides), examples include:poly(chlorostyrene), poly(bromostyrene), and poly(fluorostyrene).Examples of poly(alkoxy styrene) include: poly(methoxy styrene), andpoly(ethoxy styrene). Among these examples, as particularly preferablestyrene group polymers, are: polystyrene, poly(p-methyl styrene),poly(m-methyl styrene), poly(p-tertiary butyl styrene),poly(p-chlorostyrene), poly(m-chloro styrene), poly(p-fluoro styrene),and copolymers of styrene and p-methyl styrene may be mentioned.Furthermore, as comonomers of syndiotactic vinyl-aromatic groupcopolymers, besides monomers of above explained styrene group polymer,olefin monomers such as ethylene, propylene, butene, hexene, or octene;diene monomers such as butadiene, isoprene; polar vinyl monomers such ascyclic diene monomer, methyl methacrylate, maleic acid anhydride, oracrylonitrile may be mentioned.

Aliphatic copolyesters and aliphatic polyamides may also be usefulmaterials for strippable boundary layers. As for polyester polymers andcopolymers, the diacids can be chosen from terephthalic acid,isophthalic acid, phthalic acid, all isomeric naphthalenedicarboxylicacids (2,6-, 1,2-, 1,3-, 1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-,2,7-, and 2,8-), bibenzoic acids such as 4,4′-biphenyl dicarboxylic acidand its isomers, trans-4,4′-stilbene dicarboxylic acid and its isomers,4,4′-diphenyl ether dicarboxylic acid and its isomers,4,4′-diphenylsulfone dicarboxylic acid and its isomers,4,4′-benzophenone dicarboxylic acid and its isomers, halogenatedaromatic dicarboxylic acids such as 2-chloroterephthalic acid and2,5-dichloroterephthalic acid, other substituted aromatic dicarboxylicacids such as tertiary butyl isophthalic acid and sodium sulfonatedisophthalic acid, cycloalkane dicarboxylic acids such as1,4-cyclohexanedicarboxylic acid and its isomers and2,6decahydronaphthalene dicarboxylic acid and its isomers, bi- ormulti-cyclic dicarboxylic acids (such as the various isomeric norbornaneand norbornene dicarboxylic acids, adamantane dicarboxylic acids, andbicyclo-octane dicarboxylic acids), alkane dicarboxylic acids (such assebacic acid, adipic acid, oxalic acid, malonic acid, succinic acid,glutaric acid, azelaic acid, and dodecane dicarboxylic acid.), and anyof the isomeric dicarboxylic acids of the fused-ring aromatichydrocarbons (such as indene, anthracene, pheneanthrene, benzonaphthene,fluorene and the like). Alternatively, alkyl esters of these monomers,such as dimethyl terephthalate, may be used.

Suitable diol comonomers include but are not limited to linear orbranched alkane diols or glycols (such as ethylene glycol, propanediolssuch as trimethylene glycol, butanediols such as tetramethylene glycol,pentanediols such as neopentyl glycol, hexanediols,2,2,4-trimethyl-1,3-pentanediol and higher diols), ether glycols (suchas diethylene glycol, triethylene glycol, and polyethylene glycol),chain-ester diols such as3hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate,cycloalkane glycols such as 1,4-cyclohexanedimethanol and its isomersand 1,4-cyclohexanediol and its isomers, bior multicyclic diols (such asthe various isomeric tricyclodecane dimethanols, norbornane dimethanols,norbornene dimethanols, and bicyclo-octane dimethanols), aromaticglycols (such as 1,4-benzenedimethanol and its isomers, 1,4-benzenedioland its isomers, bisphenols such as bisphenol A, 2,2′-dihydroxy biphenyland its isomers, 4,4′dihydroxymethyl biphenyl and its isomers, and1,3-bis(2-hydroxyethoxy)benzene and its isomers), and lower alkyl ethersor diethers of these diols, such as dimethyl or diethyl diols.

In the exemplary embodiment where at least one boundary layer isdesigned to permanently adhere to at least one adjacent optical film,the constituent materials must be provided with sufficient adhesion tothat adjacent optical film. These materials will be chosen with regardto their adhesive properties to an optical film and, optionally, toanother component of the boundary layer (in case of a multilayerboundary layer). Some materials that may be useful are those listedabove and polymers of the same classes that have been modified to adhereto an optical film.

In some exemplary embodiments, the strippable boundary layer(s) mayinclude low melting and low crystallinity polypropylenes and theircopolymers; low melting and low crystallinity polyethylenes and theircopolymers, low melting and low crystallinity polyesters and theircopolymers, or any suitable combination thereof. Such low melting andlow crystalinity polypropylenes and their copolymers consist ofpropylene homopolymers and copolymers of propylene and ethylene oralpha-olefin materials having between 4 to 10 carbon atoms. The term“copolymer” includes not only the copolymer, but also terpolymers andpolymers of four or more component polymers. Suitable low melting andlow crystallinity polypropylenes and their copolymers include, forexample, syndiotactic polypropylene (such as, Finaplas 1571 from TotalPetrochemicals, Inc.), which is a random copolymer with an extremely lowethylene content in the syndiotactic polypropylene backbone, and randomcopolymers of propylene (such as PP8650 or PP6671 from Atofina, which isnow Total Petrochemicals, Inc.). The described copolymers of propyleneand ethylene can also be extrusion blended with homopolymers ofpolypropylene to provide a higher melting point strippable boundarylayer if needed.

Other suitable low melting and low crystallinity polyethylenes andpolyethylene copolymers include, for example, linear low-densitypolyethylene and ethylene vinyl alcohol copolymers. Suitablepolypropylenes include, for example, random copolymers of propylene andethylene (for example, PP8650 from Total Petrochemicals, Inc.), orethylene octene copolymers (for example, Affinity PT 1451 from DowChemical Company). In some embodiments of the present disclosure, thecontinuous phase includes an amorphous polyolefin, such as an amorphouspolypropylene, amorphous polyethylene, an amorphous polyester, or anysuitable combination thereof or with other materials. In someembodiments, the materials of the strippable boundary layers can includenucleating agents, such as sodium benzoate to control the rate ofcrystallization. Additionally, anti-static materials, anti-blockmaterials, coloring agents such as pigments and dyes, polarizing dyes,migratory lubricants, stabilizers and other processing aids may beadded. Additionally or alternatively, the rough strippable skin layersmay include any other appropriate material. In some exemplaryembodiments, migratory antistatic agents can be used in the strippableboundary layers to lower their adhesion to the optical films.

Rough Strippable Boundary Layers

In the exemplary embodiments of the present disclosure that include atleast one rough strippable boundary layer, the boundary layer or layersmay include any materials described above or any combination thereof.For example, the continuous phase or one of the first and secondimmiscible polymers may include any material mentioned in reference tothe strippable boundary layers described above.

The degree of surface roughness of the rough strippable boundary layerscan be adjusted by mixing or blending different materials, for example,polymeric materials, inorganic materials, or both into the dispersephase. In addition, the ratio of disperse phase to continuous phase canbe adjusted to control the degree of surface roughness and adhesion andwill depend on the particular materials used. Thus, in the exemplaryembodiments including a rough strippable boundary layer, one, two ormore polymers would function as the continuous phase, while one, two ormore materials, which may or may not be polymeric, would provide adisperse phase with a suitable surface roughness for imparting a surfacetexture. The one or more polymers of the continuous phase can beselected to provide a desired adhesion to the material of the opticalfilm. A material with relatively high crystallinity, such as highdensity polyethylene (HDPE) or polycaprolactone, can be blended into therough strippable boundary layers in order to impart rough texture intothe surface of an optical film that is adjacent to the rough strippableboundary layer and to affect adhesion. For example, HDPE could beblended into low crystallinity syndiotactic polypropylene (sPP) forimproving surface roughness along with a low crystallinity poly(ethyleneoctene) (PE-PO) for improving adhesion.

Where the disperse phase is capable of crystallization, the roughness ofthe strippable skin layer or layers can be enhanced by crystallizationof this phase at an appropriate extrusion processing temperature, degreeof mixing, and quenching, as well as through addition of nucleationagents, such as aromatic carboxylic-acid salts (sodium benzoate);dibenzylidene sorbitol (DBS), such as Millad 3988 from Milliken &Company; and sorbitol acetals, such as Irgaclear clarifiers by CibaSpecialty Chemicals and NC-4 clarifier by Mitsui Toatsu Chemicals. Othernucleators include organophosphate salts and other inorganic materials,such as ADKstab NA-11 and NA-21 phosphate esters from Asahi-Denka andHyperform HPN-68, a norbornene carboxylic-acid salt from Milliken &Company. In some exemplary embodiments, the disperse phase includesparticles, such as those including inorganic materials, that willprotrude from the surface of the rough strippable boundary layers andimpart surface structures into the optical film when the optical body isextruded, oriented, laminated or stretched.

The disperse phase of the rough strippable boundary layers can includeparticles or other rough features that are sufficiently large (forexample, at least 0.1 micrometers average diameter) to be used to imparta surface texture into the outer surface of an adjacent layer of theoptical film. At least a substantial portion of protrusions of thedisperse phase should typically be larger than the wavelength of thelight it is illuminated with but still small enough not to be resolvedwith an unaided eye. Such particles can include particles of inorganicmaterials, such as silica particles, talc particles, sodium benzoate,calcium carbonate, a combination thereof or any other suitableparticles. Alternatively, the disperse phase can be formed frompolymeric materials that are (or become) substantially immiscible in thecontinuous phase under the appropriate conditions.

The disperse phase can be formed from one or more materials, such asinorganic materials, polymers, or both that are different from at leastone polymer of the continuous phase and immiscible therein, with thedisperse polymer phases having typically a higher degree ofcrystallinity than the polymer or polymers of the continuous phase. Itis preferred that the disperse phase is only mechanically miscible orimmiscible with the continuous phase polymer or polymers. The dispersephase material or materials and the continuous phase material ormaterials can phase separate under appropriate processing conditions andform distinct phase inclusions within the continuous matrix, andparticularly at the interface between the optical film and the roughstrippable skin layer.

Exemplary polymers that are particularly suitable for use in thedisperse phase include styrene acrylonitrile, modified polyethylene,polycarbonate and copolyester blend, ε-caprolactone polymer, such asTONE™ P-787, available from Dow Chemical Company, random copolymer ofpropylene and ethylene, other polypropylene copolymers, poly(ethyleneoctene) copolymer, anti-static polymer, high density polyethylene,medium density polyethylene, linear low density polyethylene andpolymethyl methacrylate. The disperse phase may include any otherappropriate material, such as any suitable crystallizing polymer and itmay include the same materials as one or more of the materials used inthe optical film.

In some exemplary embodiments, the strippable boundary layer or layersmay include at least 3 materials for the purposes of controllingstrippable layer adhesion and providing a higher surface featuredensity. In some exemplary embodiments, more than 2 disperse sub-phasescan result in rough features or protrusions of different sizes orcompounded protrusions, such as “protrusion-on-protrusion”configurations, i.e., impart smaller concave surface features(depressions) between larger concave surface features (depressions),and, in some exemplary embodiments, smaller concave surface features(depressions) within larger concave surface features (depressions). Suchconstructions can be beneficial for creating hazier surfaces on opticalfilms.

Materials used in such exemplary embodiments are available fromdifferent manufacturers as described: PEN (0.48 IV PEN from 3M Company),SAN (Tyril 880 from Dow Chemical), sPP (1571 available from Atofina, nowTotal Petrochemicals, Inc.), MDPE (Marflex TR130 available fromChevron-Philips), Admer (SE810 available from Mitsui Petrochemicals,Inc.), Xylex (Xylex 7200 available from GE Plastics Inc.), randompropylene-ethylene copolymer (PP8650 available from Atofina, now TotalPetrochemicals, Inc.), Pelestat 300 (Pelestat 300 available from TomenAmerica), Pelestat 6321 (Pelestat 6321 available from Tomen America),polycaprolactone (Tone 787), PMMA (VO44 available from Atofina, nowTotal Petrochemicals, Inc. Chemical), Polystyrene (Styron 685 availablefrom Dow Chemical Company).

Material Compatibility and Methods

Optical bodies of the present disclosure can be made, for example, bycoextrusion using a feedblock method. Exemplary manufacturing processesare described, for example, in U.S. patent Ser. Nos. 09/229,724,08/402,041, 09/006,288 and U.S. Patent Application Publication No.2001/0013668, U.S. Pat. No. 6,352,761, which are hereby incorporatedherein by reference. Preferably, the materials of the optical bodies,and in some exemplary embodiments, of the first optical layers, thesecond optical layers, the optional non-optical layers, and of thestrippable boundary layers are chosen to have similar rheologicalproperties (e.g., melt viscosities) so that they can be co-extrudedwithout flow instabilities. The effect of shear forces duringcoextrusion can be reduced by coextruding one or more outer skin layerswhen forming the optical bodies of the present disclosure. The materialsof the outer skin layer or layers can be selected so that these layersmay be removed from the optical body after or prior to any processingstep.

The optical body exiting the feedblock manifold can then enter a shapingunit, such as a die. Alternatively, prior to entering the shaping unit,the polymeric stream may be split to form two or more streams that maythen be recombined by stacking. This process is usually referred to asmultiplication. Exemplary multipliers are described, for example, inU.S. Pat. Nos. 5,094,788 and 5,094,793, incorporated by referenceherein. Strippable boundary layers may be added to the optical bodies ofthe present disclosure during coextrusion of the optical layers oroptical film or after coextrusion of the optical layers or optical film,for example, prior to multiplication. In some exemplary embodiments,different strippable boundary layers may be added at different stages ofthe production process. After the optical body is discharged from theshaping unit, it may be cast onto a chill roll, casting wheel or castingdrum.

Subsequently, the optical body may be drawn or stretched to produce thefinished article. Depending on the type of optical films included intothe optical body, the drawing or stretching may be accomplished in one,two or more steps. Where one or more of the optical films included intoan optical body of the present disclosure is a reflective polarizer, theoptical body may be drawn uniaxially or substantially uniaxially in thetransverse direction (TD), while allowed to relax in the machinedirection (MD) as well as the normal direction (ND). Suitable methodsand apparatuses that can be used to draw such exemplary embodiments ofthe present disclosure are described in U.S. Application PublicationNos. 2002/0190406, 2002/0180107, 2004/0099992 and 2004/0099993, thedisclosures of which are hereby incorporated by reference herein.

Drawing Optical Bodies in Uniaxial or Substantially Uniaxial Manner

The processes of the present disclosure may include stretching anoptical body that can be described with reference to three mutuallyorthogonal axes corresponding to the machine direction (MD), thetransverse direction (TD), and the normal direction (ND). These axescorrespond to the width (W), length (L), and thickness (T) of theoptical body 200 illustrated in FIG. 4. The stretching process stretchesa region 200 of the optical body from an initial configuration 240 to afinal configuration 260. The machine direction is the general directionalong which the film travels through a stretching device, for example,the apparatus illustrated in FIG. 5. The transverse direction (TD) isthe second axis within the plane of the film and is orthogonal to themachine direction (MD). The normal direction (ND) is orthogonal to bothMD and TD and corresponds generally to the thickness dimension of thepolymer film.

FIG. 5 illustrates one embodiment of a stretching apparatus and methodof the present disclosure. The optical body can be provided to thestretching apparatus by any desirable method. For example, the opticalbody can be produced in a roll or other form and then provided tostretching apparatus. As another example, the stretching apparatus canbe configured to receive the optical body from an extruder (if, forexample, the optical body is generated by extrusion and ready forstretching after extrusion) or a coater (if, for example, the opticalbody is generated by coating or is ready for stretching after receivingone or more coated layers) or a laminator (if, for example the opticalbody is generated by lamination or is ready for stretching afterreceiving one or more laminated layers).

Generally, an optical body 140 is presented in region 130 to one or moregripping members that are configured and arranged to hold opposing edgesof the optical body and convey the optical body along opposing tracks164 defining predetermined paths. The gripping members (not shown)typically hold the optical body at or near its edges. The portions ofthe optical body held by the gripping members are often unsuitable foruse after stretching so the position of the gripping members istypically selected to provide sufficient grip on the film to permitstretching while controlling the amount of waste material generated bythe process.

Gripping members, such as clips, can be directed along the track by, forexample, rollers 162 rotating a chain along the track with the grippingmembers coupled to the chain. The rollers are connected to a drivermechanism that controls the speed and direction of the film as it isconveyed through the stretching apparatus. Rollers can also be used torotate and control the speed of belt-type gripping members.

Returning further to FIG. 5, the apparatus optionally includes apreconditioning region 132 that typically is enclosed by an oven 154 orother apparatus or arrangement to heat the optical body in preparationfor stretching. The preconditioning region can include a preheating zone142, a heat soak zone 144, or both.

The optical film may be stretched in the primary stretching region 134.Typically, within the primary stretching region 134 the optical body isheated or maintained in a heated environment above the glass transitionof the polymer(s) of the optical body. Within the primary stretchingregion 134, the gripping members follow generally diverging tracks tostretch the optical body by a desired amount. The tracks in the primarystretching region and in other regions of the apparatus can be formedusing a variety of structures and materials. Outside of the primarystretching region, the tracks are typically substantially linear. Theopposing linear tracks can be parallel or can be arranged to beconverging or diverging. Within the primary stretching region, thetracks are generally diverging.

In all regions of the stretching apparatus, the tracks can be formedusing a series of linear or curvilinear segments that are optionallycoupled together. As an alternative or in particular regions or groupsof regions, the tracks can be formed as a single continuousconstruction. In at least some embodiments, the tracks in the primarystretching region are coupled to, but separable from, the tracks of thepreceding regions. The tracks 1140, 1141 in the succeedingpost-conditioning or removal regions are typically separated from thetracks of the primary stretching region, as illustrated in FIG. 5. Insome embodiments, the positions of one or more, and preferably all, ofthe track segments are adjustable (e.g., pivotable about an axis) sothat the overall shape of the tracks can be adjusted if desired.Continuous tracks can also be used through each of the regions.

Typically, the portions of the optical body that were held by thegripping members through the primary stretching region are removed. Tomaintain a substantially uniaxial draw throughout substantially all ofthe draw history (as shown in FIG. 5), at the end of the transversestretch, the rapidly diverging edge portions 156 are preferably severedfrom the stretched optical body 148 at a slitting point 158. A cut canbe made at 158 and flash or unusable portions 156 can be discarded.

Release of the selvages from a continuous gripping mechanism can be donecontinuously; however, release from discrete gripping mechanisms, suchas tenter clips, should preferably be done so that all the materialunder any given clip is released at once. This discrete releasemechanism may cause larger upsets in stress that may be felt by thedrawing web upstream. In order to assist the action of the isolatingtakeaway device, it is preferred to use a continuous selvage separationmechanism in the device, e.g. the “hot” slitting of the selvage from thecentral portion of a heated, drawn film.

The slitting location is preferably located near enough to the“gripline”, e.g. the isolating takeaway point of first effective contactby the gripping members of the take-away system, to minimize or reducestress upsets upstream of that point. If the film is slit before thefilm is gripped by the take-away system, instable takeaway can result,for example, by film “snapback” along TD. The film is thus preferablyslit at or downstream of the gripline. Slitting is a fracture processand, as such, typically has a small but natural variation in spatiallocation. Thus it may be preferred to slit slightly downstream of thegripline to prevent any temporal variations in slitting from occurringupstream of the gripline. If the film is slit substantially downstreamfrom the gripline, the film between the takeaway and boundary trajectorywill continue to stretch along TD. Since only this portion of the filmis now drawing, it now draws at an amplified draw ratio relative to theboundary trajectory, creating further stress upsets that could propagateupstream, for example, undesirable levels of machine direction tensionpropagating upstream.

The slitting is preferably mobile and re-positionable so that it canvary with the changes in takeaway positions needed to accommodatevariable final transverse draw direction ratio or adjustment of theposition of the take-away system. An advantage of this type of slittingsystem is that the draw ratio can be adjusted while maintaining the drawprofile simply by moving the take-away slitting point 158, preferablyalong the MD. A variety of slitting techniques can be used including aheat razor, a hot wire, a laser, a focused beam of intense IR radiationor a focused jet of heated air.

The apparatus shown in FIG. 5 may optionally include a post-conditioningregion 136. For example, the optical body may be set in zone 148 andquenched in zone 150. A takeaway system may be used to remove theoptical body from the primary stretching region 134. In the illustratedembodiment, this takeaway system is independent of (i.e., isolated fromor not directly connected to) the tracks upon which the film wasconveyed through the primary stretching region. The takeaway system canuse any film conveyance structures such as tracks 1140, 1141 withgripping members such as, for example, opposing sets of belts or tenterclips.

In some embodiments, TD shrinkage control can be accomplished usingtracks 1140, 1141 that are angled with respect to each other. Forexample, the tracks of the take-away system can be positioned to followa slowly converging path (making an angle of no more than about 5°)through at least a portion of the post conditioning region to allow forTD shrinkage of the film with cooling. In other embodiments, the twoopposing tracks can be diverging typically at an angle of no more thanabout 3° although wider angles can be used in some embodiments. This canbe useful to increase the MD tension of the film in the primarystretching region to, for example, reduce property non-uniformity suchas the variation of principal axes of refractive index across the film.

In some exemplary embodiments, the centerline of the take-away system isangled with respect to the centerline of the film as the film isconveyed through the tracks 164 of the primary stretching region. Anangled take-away system, primary stretching zone, or both can be usefulto provide films where the principal axis or axes of an property of thefilm, such as the refractive index axes or tear axis, is angled withrespect to the film. In some embodiments, the angle that the take-awaysystem makes with respect to the primary stretching zone is adjustablemanually or mechanically using a computer-controlled driver or othercontrol mechanism or both.

The exemplary process of FIG. 5 also includes a removal portion inregion 138. Optionally a roller 165 may be used to advance the stretchedfilm 152, but this component may be omitted if desired. Another cut 160may be made and unused portion 161 may be discarded. Film leaving thetake-away system is typically wound on rolls for later use.Alternatively, direct converting may take place after take away.

The paths defined by the opposing tracks affect the stretching of thefilm in the MD, TD, and ND directions. The stretching (or drawing)transformation can be described as a set of draw ratios: the machinedirection draw ratio (MDDR), the transverse direction draw ratio (TDDR),and the normal direction draw ratio (NDDR). When determined with respectto the film, the particular draw ratio is generally defined as the ratioof the current size (for example, length, width, or thickness) of thefilm in a desired direction (for example, TD, MD, or ND) and the initialsize (for example, length, width, or thickness) of the film in that samedirection. At any given point in the stretching process, TDDRcorresponds to a ratio of the current separation distance of theboundary trajectories, L, and the initial separation distance of theboundary trajectories, L, at the start of the stretch. In other words,TDDR=L/L₀=λ. Some useful values of TDDR include about 1.5 to about 7 ormore. Exemplary useful values of TDDR include about 2, 4, 5 and 6. Otherexemplary useful values of TDDR lie in the ranges of about 4 to about20, about 4 to about 12, about 4 to about 20, about 4 to about 8 andabout 12 to about 20.

As explained in U.S. Application Publication Nos. 2002/0190406,2002/0180107, 2004/0099992 and 2004/0099993, substantially uniaxialdrawing conditions, with an increase in dimension in the transversedirection, result in TDDR, MDDR, and NDDR approaching λ, (λ)^(−1/2), and(λ)^(−1/2), respectively, assuming constant density of the material. Aperfectly uniaxially oriented film is one in whichMDDR=(NDDR)^(−1/2)=(TDDR)^(−1/2) throughout the draw.

A useful measure of the extent of uniaxial character, U, can be definedas: $U = \frac{\frac{1}{MDDR} - 1}{{TDDR}^{1/2} - 1}$For a perfect uniaxial draw, U is one throughout the draw. When U isless than one, the drawing condition is considered “subuniaxial”. When Uis greater than one, the drawing condition is considered“super-uniaxial”. States of U greater than unity represent variouslevels of over-relaxing. These over-relaxed states produce MDcompression from the boundary edge. U can be corrected for changes indensity to give U_(f) according to the following formula:$U_{f} = \frac{\frac{1}{MDDR} - 1}{( \frac{TDDR}{\rho_{f}} )^{1/2} - 1}$

In some exemplary embodiments, the film is drawn in plane (i.e., theboundary trajectories and tracks are coplanar) such as shown in FIG. 5,but non-coplanar stretching trajectories are also within the scope ofthe present disclosure. With in-plane boundary trajectories, the resultfor a perfect uniaxial orientation is a pair of mirror symmetric,in-plane, parabolic trajectories diverging away from the in-plane MDcenterline.

Uniaxial draw may be maintained along the entire history of the draw aslong as the speed of the central point reduces at every point along thecentral trace from its initial speed by a factor of exactly the squareroot of the reciprocal of the instantaneous TDDR measured between thecorresponding opposing points on the opposing boundary trajectories.

Various factors can affect the ability to achieve uniaxial orientationincluding, for example, non-uniform thickness of the polymer film,non-uniform heating of the polymer film during stretching, and theapplication of additional tension (for example, machine directiontension) from, for example, down-web regions of the apparatus. However,in many instances it is not necessary to achieve perfect uniaxialorientation. In some exemplary implementations of the presentdisclosure, any value of U>0 may be useful. Instead, a minimum orthreshold U value or an average U value that is maintained throughoutthe draw or during a particular portion of the draw can be defined. Forexample, in some exemplary embodiments, an acceptable minimum/thresholdor average U value can be 0.2, 0.5, 0.7, 0.75, 0.8, 0.85, 0.9, or 0.95,as desired, or as needed for a particular application. When a specificvalue of U is chosen, the equations above provide a specificrelationship between MDDR and TDDR, which, when coupled with otherrelevant considerations, specify a broader class of boundarytrajectories that also include the parabolic trajectories for Uapproaching unity. Trajectories that exhibit values of U below unity forat least a final portion of the draw are referred to herein assub-parabolic trajectories.

The classes of trajectories described above are illustrative and shouldnot be construed as limiting. A host of trajectory classes areconsidered to lie within the scope of the present invention. The primarystretching region can contain two or more different zones with differentstretching conditions. For example, one trajectory from a first class oftrajectories can be selected for an initial stretching zone and anothertrajectory from the same first class of trajectories or from a differentclass of trajectories can be selected for each of the subsequentstretching zones.

Although the present disclosure encompasses all boundary trajectoriescomprising a minimum value of U>0, typical embodiments of the presentdisclosure include all substantially uniaxial boundary trajectoriescomprising a minimum value of U of about 0.2, about 0.5, preferablyabout 0.7, more preferably about 0.75, still more preferably about 0.8and even more preferably about 0.85. The minimum U constraint may beapplied over a final portion of the draw defined by a critical TDDRpreferably of about 2.5, still more preferably about 2.0 and morepreferably about 1.5. In some embodiments, the critical TDDR may be 4, 5or more. Above a critical TDDR, certain materials, e.g. certainmonolithic and multilayer films comprising orientable and birefringentpolyesters, may begin to lose their elasticity or capability of snapback, e.g. because of the development of structure such asstrain-induced crystallinity.

As an example of acceptable substantially uniaxial applications, theoff-angle characteristics of reflective polarizers are strongly impactedby the difference in the MD and ND indices of refraction when TD is theprincipal mono-axial draw direction. An index difference in MD and ND of0.08 is acceptable in some applications. A difference of 0.04 isacceptable in others. In more stringent applications, a difference of0.02 or less is preferred. For example, the extent of uniaxial characterof 0.85 is sufficient in many cases to provide an index of refractiondifference between the MD and ND directions in polyester systemscontaining polyethylene naphthalate (PEN) or copolymers of PEN of 0.02or less at 633 nm for mono-axially transverse drawn films. For somepolyester systems, such as polyethylene terephthalate (PET), a lower Uvalue of 0.80 or even 0.75 may be acceptable because of lower intrinsicdifferences in refractive indices in non-substantially uniaxially drawnfilms.

For sub-uniaxial draws, the final extent of truly uniaxial character canbe used to estimate the level of refractive index matching between the y(MD) and z (ND) directions by the equationΔn _(yz) =Δn _(yz)(U=0)×(1−U)

where Δn_(yz) is the difference between the refractive index in the MDdirection (i.e., y-direction) and the ND direction (i.e., z-direction)for a value U and Δn_(yz)(U=0) is that refractive index difference in afilm drawn identically except that MDDR is held at unity throughout thedraw. This relationship has been found to be reasonably predictive forpolyester systems (including PEN, PET, and copolymers of PEN or PET)used in a variety of optical films. In these polyester systems,Δn_(yz)(U=0) is typically about one-half or more the differenceΔn_(xy)(U=0), which is the refractive difference between the twoin-plane directions MD (y-axis) and TD (x-axis). Typical values forΔn_(xy)(U=0) range up to about 0.26 at 633 nm. Typical values forΔn_(yz)(U=0) range up to 0.15 at 633 nm. For example, a 90/10 coPEN,i.e. a copolyester comprising about 90% PEN-like repeat units and 10%PET-like repeat units, has a typical value at high extension of about0.14 at 633 nm. Films comprising this 90/10 coPEN with values of U of0.75, 0.88 and 0.97 as measured by actual film draw ratios withcorresponding values of Δn_(yz) of 0.02, 0.01 and 0.003 at 633 nm havebeen made according to the methods of the present invention.

A variety of other boundary trajectories are available when U issubuniaxial at the end of the stretching period. In particular, usefulboundary trajectories include coplanar trajectories where TDDR is atleast 5, U is at least 0.7 over a final portion of the stretch afterachieving a TDDR of 2.5, and U is less than 1 at the end of the stretch.Other useful trajectories include coplanar and non-coplanar trajectorieswhere TDDR is at least 7, U is at least 0.7 over a final portion of thestretch after achieving a TDDR of 2.5, and U is less than 1 at the endof the stretch. Useful trajectories also include coplanar andnon-coplanar trajectories where TDDR is at least 6.5, U is at least 0.8over a final portion of the stretch after achieving a TDDR of 2.5, and Uis less than 1 at the end of the stretch. Useful trajectories includecoplanar and non-coplanar trajectories where TDDR is at least 6, U is atleast 0.9 over a final portion of the stretch after achieving a TDDR of2.5, and U is less than 1 at the end of the stretch. Useful trajectoriesalso include coplanar and non-coplanar trajectories where TDDR is atleast 7 and U is at least 0.85 over a final portion of the stretch afterachieving a TDDR of 2.5.

Generally, various methods may be used for forming and processingoptical bodies of the present disclosure, which may include extrusionblending, coextrusion, film casting and quenching, lamination andorientation, such as uniaxial and biaxial (balanced or unbalanced)stretching. As stated above, the optical bodies can take on variousconfigurations, and thus the methods vary depending upon theconfiguration and the desired properties of the final optical body.

Thus, the present disclosure provides optical bodies includingstrippable boundary layers and methods for producing such optical bodiesthat could significantly increase production capacity and decrease laborcosts, because at least twice as much product can be stretchedconcurrently. Converting costs also can be reduced, because eachconverted piece will yield at least two parts of the film product. Theresulting optical body can be left intact during shipment and handlinguntil a customer is ready to use the films. This allows one or moresurfaces of the optical film to be protected by the adjacent boundarylayer.

Although the present invention has been described with reference to theexemplary embodiments specifically described herein, those of skill inthe art will recognize that changes may be made in form and detailwithout departing from the spirit and scope of the present disclosure.

1. An optical body comprising a first optical film, a second opticalfilm and one or more strippable boundary layers disposed between thefirst and second optical films such that each major surface of astrippable boundary layer is disposed adjacent to an optical film oranother strippable boundary layer, wherein at least one of the first andsecond optical films comprises a reflective polarizer.
 2. The opticalbody of claim 1, wherein the reflective polarizer is a diffusereflective polarizer comprising a disperse polymeric phase and acontinuous polymeric phase.
 3. The optical body of claim 2, wherein thereflective polarizer has a thickness of no more than 1 mil.
 4. Theoptical body of claim 2, wherein at least one of the disperse orcontinuous phase has an in-plane birefringence of at least 0.1.
 5. Theoptical body of claim 1, further comprising a third optical film and atleast one additional strippable boundary layer disposed between thethird optical film and the second optical film.
 6. The optical body ofclaim 1, wherein the reflective polarizer is a multilayer reflectivepolarizer comprising a plurality of first and second optical layers. 7.The optical body of claim 6, wherein the reflective polarizer has nomore than optical layers.
 8. The optical body of claim 6, wherein atleast one of the first and second optical layers has an in-planebirefringence of at least 0.1.
 9. The optical body of claim 1, wherein astrippable boundary layer comprises at least one of: a fluropolymer, apolypropylene, a modified polypropylene, an aliphatic polyolefin, apolyethylene, a polyethylene copolymer, polymethylpentene, a cyclicolefin copolymer, a syndiotactic polymer, an atactic vinyl aromaticpolymer, a polysyrene, and a copolymer of styrene.
 10. The optical bodyof claim 1, further comprising at least one outer skin layer.
 11. Amethod for processing an optical body, comprising: providing an opticalbody comprising a first optical film, a second optical film and at leastone strippable boundary layer disposed between the first and secondoptical films; conveying the optical body into a stretching region;stretching the optical body to increase a transverse dimension of theoptical body while conveying the opposing edges of the optical bodyalong generally diverging paths in a machine direction, wherein thegenerally diverging paths are configured and arranged to provide amachine direction draw ratio (MDDR), a normal direction draw ratio(NDDR) and a transverse direction draw ratio (TDDR) that approach thefollowing relationship:MDDR=NDDR=(TDDR)^(−1/2) during the stretching.
 12. The method of claim11, wherein the diverging paths are substantially parabolic.
 13. Themethod of claim 11, wherein the diverging paths are linearapproximations of substantially parabolic paths.
 14. The method of claim11, wherein the diverging paths are coplanar.
 15. The method of claim11, wherein in the stretched optical body at least one of the first andsecond optical films comprises a reflective polarizer.
 16. The method ofclaim 11, wherein stretching the film comprises stretching the opticalbody to a draw ratio in excess of four.
 17. The method of claim 11,wherein the step of stretching comprises moving the opposing edgeportions along diverging paths that are substantially symmetrical abouta center axis of the optical body.
 18. The method of claim 11, furthercomprising providing the optical body to the stretcher in a continuousmanner from a roll of film.
 19. The method of claim 11, furthercomprising coextruding the optical body in-line with stretching.
 20. Themethod of claim 20, wherein coextruding the optical body comprisesmultiplication and the at least one boundary layer is added prior tomultiplication.
 21. The method of claim 11, wherein the stretched filmcomprises at least one material with indices of refraction in a lengthdirection corresponding to the machine direction and a thicknessdirection that are substantially the same but substantially differentfrom an index of refraction in a width direction.
 22. The method ofclaim 11, wherein the minimum value of the extent of uniaxial character,U, is at least 0.7, wherein U is defined asU=(1/MDDR−1)/(TDDR ^(1/2)−1).
 23. A method of processing an opticalbody, the method comprising: providing an optical body comprising afirst optical film, a second optical film and at least one strippableboundary layer disposed between the first and second optical films;conveying the optical body within a stretcher along a machine directionwhile holding opposing edge portions of the optical body; and stretchingthe optical body to a draw ratio in excess of four within the stretcherby moving the opposing edge portions along diverging non-linear paths,wherein, during the stretching, the minimum value of the extent ofuniaxial character, U, is at least 0.7 over a final portion of thestretching after achieving a TDDR of 2.5 and U is less than 1 at the endof the stretching, wherein U is defined asU=(1/MDDR−1)/(TDDR ^(1/2)−1) wherein MDDR is the machine direction drawratio and TDDR is the transverse direction draw ratio as measuredbetween the diverging paths.
 24. The method of claim 24, wherein theminimum value of the extent of uniaxial character is at least 0.8. 25.The method of claim 24, wherein the extent of uniaxial character, U, isat least 0.8 over a final portion of the stretching after achieving aTDDR of 2.0.
 26. The method of claim 24, wherein at least one of thefirst and second optical films comprises a multilayer film having aplurality of alternating layers of different polymeric composition. 27.A method of processing an optical body, the method comprising: providingan optical body comprising a first optical film, a second optical filmand at least one strippable boundary layer disposed between the firstand second optical films; conveying the optical body within a stretcheralong a machine direction while holding opposing edge portions of theoptical body; and stretching the optical body within the stretcher bymoving the opposing edge portions along diverging non-linear paths,wherein, during the stretching of the optical body, the speed of thefilm along the machine direction decreases by a factor of approximatelyλ^(1/2) where λ is the transverse direction draw ratio.
 28. The methodof claim 28, wherein at least one of the first and second optical filmcomprises a multilayer film having a plurality of alternating layers ofdifferent polymeric composition.